Pumped evacuated tube water hammer pile driver method

ABSTRACT

Driving piles with a liquid ram or spear generated in a pumped, evacuated tube. Various configurations are shown, including those in which the pile itself defines at least a portion of the working chamber for generating water hammer, and others in which the working chamber is defined at least in part by tubes other than the pile.

United States Patent Wisotsky Dec. 2, 1975 PUMPED EVACUATED TUBE WATER HAMMER PILE DRIVER METHOD Primary ExaminerCasmir A. Nunberg H Assistant ExaminerDavid H. Corbin [75] Inventor. Serge S. Wlsotsky, Sharon, Mass. Attorney, Agent or Firm Roben R. y q [73] Assignee: Orb, Inc., Marion, Ohio [22] Filed: July 31, 1974 Appl. No.: 493,421

Related U.S. Application Data [57] ABSTRACT Driving piles with a liquid ram or spear generated in a pumped, evacuated tube. Various configurations are shown, including those in which the pile itself defines at least a portion of the working chamber for generating water hammer, and others in which the working chamber is defined at least in part by tubes other than the pile.

4 Claims, 11 Drawing Figures U.S. Patent Dec. 2, 1975 Sheet 1 of 5 3,922,869

FIGI U.S. Patent Dec. 2, 1975 Sheet 2 of 5 3,922,869

Patent Dec. 2, 1975 Sheet 3 of5 3,922,869

U.S. Patent Dec. 2, 1975 Sheet 4 of5 3,922,869

HGIO

FIG?) US. Patent Dec. 2, 1975 Sheet 5 of5 3,922,869

PUMPED EVACUATED TUBE WATER HAMMER PILE DRIVER METHOD CROSS-REFERENCE This application is a division of application Ser. No. 267.741 filed June 30, 1972 which application was a continuation in part of prior copending application Ser. No. 163,422, filed July 16, 1971, the disclosure of which prior copending application is hereby incorporated by reference.

BAC KG ROUND The kinetic energy output of a pile driver is a function of its driving mass and its velocity at the instant of impact with a pile. The emplacement of piles in the ground by pile driving is accomplished by transmitting the kinetic energy of a hammer or other driving mass to a pile in sufficient quantity to cover nonproductive energy consuming factors such as impact stresses, radiation, reflection and ground quake, and to overcome the friction, elasticity and inertial impedence components of the pile and ground.

Increasingly larger land-based and offshore structures are constructed year after year. Larger structures demand longer and more massive piles for their foundations, more deeply embedded in the ground. This requirement is particularly severe in the case of large offshore installations such as ship terminals, and oil drilling, production and storage facilities. Without suitable foundations, such structures weighing tens of thousands of tons can be readily dislodged and toppled by heavy storms, large vessels bumping, earthquakes, and ice floes, often with catastrophic loss of life, damage to the environment and loss of invested capital. Thus, to provide adequate load-bearing and to prevent pull-out, requirements exist for driving piles hundreds of feet long, several feet in diameter, weighing hundreds of tons, and for continuing the driving to depths of soil penetration where driving resistance is severe.

A complex series of relationships pertaining to pile and soil characteristics, driving environment, economics and materials governs the design of a pile drive. However, generally speaking, the advent of piles of greater mass and conditions productive of more severe driving resistance require drivers of increasing kinetic energy output. In the absence of adequate driving energy, that which is available is consumed largely or completely by the aforementioned nonproductive energy consuming factors, leaving little or no energy to drive the pile. Under such conditions, some help is obtained by palliatives such as drilling a pilot hole, water jetting or grouting into an over-size hole, but these measures normally reduce loadbearing capacity. Thus, as each new generation of more massive piles and more severe driving conditions arises, drivers of greater energy output must be designed.

The kinetic energy output of an existing hammer can be increased by increasing either its mass or its impact velocity. The latter alternative is unattractive for a number of reasons.

First, there is the matter of the efficiency with which the hammer transfers energy to the pile. In a complete inelastic collision between a hammer and pile, the kinetic energy remaining after impact for overcoming the nonproductive factors and driving the pile is in proportion to the ratio of the hammer mass divided by the total mass of hammer plus pile. An increase in pile mass without a corresponding increase in hammer mass results in a reduction of driving efficiency.

Also, higher hammer velocities are more predisposed to produce high local impact stresses. When the latter exceed the yield point of the pile material, kinetic energy is wasted and efficiency reduced.

For these and other reasons. manufacturers discourage the use of pile driver in which the hammers mass is less than one-fourth that of the pile, and a mass ratio of one-half is generally recommended for land-based operations.

This presents a dilemma in off-shore pile driving. The largest steam hammer pile drivers currently in use in off-shore/marine work are limited, practically, by safety considerations relative to their handling in stormy weather, to weights on the order of 60 tons (hammer mass about 30 tons). Consequently, they usually are inadequate to drive the larger piles due to massmismatch.

For instance, with a 200-ton pile, the energy transfer efficiency of a 30 ton hammer would be X 30/ (30 200) or about 13%. Moreover, even this relatively small amount of energy transferred to the pile is not altogether effective in driving for other reasons stated below.

The picture is further complicated by the fact that the energy in a pile is effective to penetrate the soil only if there is a proper impedance match between the force-time-displacement characteristics of the driver and corresponding parametric thresholds of the soil. The available alternatives for varying the force-timedisplacement characteristics of a steam hammer are limited, and this presents practical problems as the tip and sides of a pile often pass through strata of widely varying characteristics as the pile penetrates the earth.

Thus, under the severest conditions, pile driving is an arduous, time consuming and expensive task which sometimes ends in failure to achieve design load-bearing capacity or depth. Also, the inability to drive large piles to sufficient depths often necessitates driving a larger number of smaller piles, so that as many as eight or sixteen piles may be required for the foundation of a single leg of a multi-leg offshore structure.

Bearing in mind the storm-weather safety considerations mentioned above it is of interest that at least one pile driver manufacturer has proposed for offshore op erations a pile driver, nominally rated at almost 500,000 ft. lb., weighing on the order of 230 tons, equivalent to the weight of several locomotives. Lifting this gigantic mass and adequately securing it during storm conditions present major challenges. Nevertheless, the fact that at least some of those active in the art seem ready to accept these formidable challenges suggests the severity of the problems and limitations with which the pile driving art is now struggling.

SUMMARY OF THE INVENTION The present invention includes apparatus comprising an evacuatable elongated closed chamber with rigid side-walls and a rigid barrier at one end. Pumping means are connected with the chamber for evacuating the chamber of at least a portion of any water, gases and vapors present therein. Valve means are provided in the chamber walls, including for instance the top of the chamber, for suddenly releasing a substantially load-velocity independent flow of water into the'evacuated portion of the chamber with its principal component of energy and motion along the longitudinal axis of the chamber toward the barrier for impacting against same directly or indirectly.

The elongated chamber may have any desired crosssection, but is conveniently circular. The side walls may thus, if desired, be defined at least by the pile itself, in which case it is also possible for the barrier to be defined by the pile tip if it is a closed pile. Or the barrier may be a fixed or moveable barrier spaced upwardly from the pile tip whether the latter is closed or open. In either case, the barrier is directly coupled to the pile. More usually, however, the chamber is defined by a closed tube separate from the pile, in which case any suitable rigid coupling means between the chamber and pile may be provided. 7

The pumping means may be a motor and mechanical impeller pump combination within the chamber. If these are secured to the outside of the chamber, the pump suction port may be connected through-the wall of the chamber to its interior. Other pump means may also be used, including pump means which dispel water by pressurizing the chamber with gas or vapor and then suck out gases, leaving the chamber at least partially evacuated. So long as the pump means is connected with the chamber, it need not'be mechanically secured thereto.

One or several of a wide variety of valve means may be employed, many examples being given in the text which follows. The valve means is so configured that it releases a flow of water, the velocity of which is substantially independent of that of the load or pile. That is, the water flows at a velocity which is substantially free of the velocity, if any, of the pile. Also, the valve means and/or chamber configuration should be such that the principal component of motion and energy of the water flow is in the desired direction. The valve inlet may connect with a water reservoir, which may or may not be pressurized, in which case the pile driver may be completely self-contained. More usually, however, the valve means inlet is in communication with the surroundings of the chamber. Thus, if the valve is deep within a flooded pile or submerged atop a submerged pile, it can draw water from its surroundings. As the water becomes deeper, the hydrostatic head provides increased operating pressures for the device.

Impacting of the water against the barrier of the evacuated tube suddenly stops the water. The resultant change of momentum creates a high powered mechanical impulse and hydraulic pressure wave of relatively high magnitude which propagates at a speed approaching that of sound in the fluid. In effect, the action is that of a fluid spear or rarn, which exerts a powerful driving force on a pile tip having a rigid driving connection with the chamber. Very large quantities of driving energy can be generated, and the pressure time characteristics of the water hammer pulses can be tailored over a wide range of values to match corresponding requirements of pile and soil characteristics.

The method of the present invention is carried out with a pile which has its tip embedded in the ground, including for instance, the sub-soil of a body of water. A closed evacuatable enclosure with sidewalls and a barrier at one end is effectively coupled with the tip of the pile for transmitting driving forces exerted upon said barrier to said tip. The method comprises: pumping out of said enclosure at least a portion of any water, gases and vapors present therein; opening valve means through the wall of said chamber; accelerating through said opening a substantially loud-velocity independent flow of water into the evacuated portion of the chamber; causing said flow to move toward said barrier with its principal component of energy and motion along the longitudinal axis of the chamber toward the barrier for impacting against same directly or indirectly; suddenly decelerating said mass against said barrier with the water substantially at its theoretical bulk modulus, thereby converting hydraulic kinetic energy to a water hammer driving pulse for driving said pile deeper into said subsoil; and repetitively evacuating, accelerating, decelerating and driving as aforesaid. Using this method, it is possible to generate powerful mechanical impulses whose force-time characteristics can be tailored over a wide range of values to better match the driving requirements of the various pile and soil conditions. Other advantages will be discussed along with certain preferred embodiments of the invention as illustrated in the accompanying drawings and text.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 13 are vertical sections and detailed views, partially broken out, showing an evacuated tube water hammer driver in which the evacuatable enclosure is a tube other than the pile itself.

FIG. 4 is a schematic diagram of a driver similar to that shown in FIG. 1, but provided with a pipe coupler and alignment means for securing the driver within a pile.-

FIG. 5 is a sectional view of a coupler for the FIG. 4 driver.

FIG. 6 is a schematic diagram of an embodiment in which the evacuatable enclosure is defined at least in part by the walls of the pile which is being driven into the subsoil.

FIGS. 7-9 are schematic diagrams of means insertable in the evacuatable enclosure of the water hammer driver for varying the water hammer impulse.

FIG. 10 is a schematic diagram of an embodiment in which the pump means is means for dispelling water by pressurizing the chamber with gas or vapor and then sucking out gases, leaving the chamber at least partially evacuated.

FIG. 11 is a schematic diagram of an embodiment of the invention in which the inlet of the valve means is connected to a pressurized reservoir.

DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with the invention, the enclosure walls can be at least partly or wholly, defined by a tube separate from the pile being driven, as disclosed by FIGS. 1 through 5, l0 and 11 hereof. FIG. 1 represents a configuration of a pile l driven underwater into the ground by a top-mounted hammer. The pile l is securely fastened to the hammer 3 by a coupling means 2. This coupling 2 may take the form of simply bolted flanges or a more sophisticated mechanical clamping arrangement similar to the scroll or pneumatically operated machine tool lathe chucks which are well-known and will not be described further. The pile hammer 3 in the present case includes hammer tube 4, made of flanged sections of heavy-walled tubing bolted together and contains a shock-mounted electric motor 5 hydraulic pump 6 combination near its bottom. (The motorpump positions may be interchanged. The pump 6 evacuates the water out of the hammer tube 4 through a center-mounted pipe 7 discharging vertically out its top. The pump 6 axially supports the discharge pipe 7 or, in other configurations, vice versa. On the top-most section of the water hammer tube 4 is mounted the fastopening water control valve 8 and its pneumatically operated actuator 9. When open, valve 8 freely admits water from the surrounding body of water through the valve body and its inlet 12 into hammer tube 4. A wire rope sling 10 supports the entire assembly from the surface and also conveys the power and control harness 11 thereto.

The configuration of the control valve 8 is detailed in FIG. 3 which shows the valve poppet 12 in the half open position. Dotted lines 13 and 14, respectively, show the valve in the fully closed and fully open positions. The two heavy dotted lines 15R and 15L represent the symmetrical water flow path through the valve into the water hammer tube 4. The center pipe 7 conducts the discharge from the pump 6 (FIG. 2) which evacuates the tube 4 of water entering the pumps suction port through the strainer intake 63 when the control valve 8 is shut. At the discharge end of the pump 6 and at the upper end of the pipe 7 are spring-loaded check valves 16L and 16U, respectively, which prevent back-flow into the water tube 4 when the discharge pressure of the evacuating pump 6 is below that of the ambient seawater hydrostatic head. A typical joint in the discharge pipe 7 is represented by the ARI type coupling 17. Concentricity of the discharge pipe 7 within the water hammer tube 4 is maintained by the split collar 18 inserted in the three-spoked spider 19 shouldered in each flanged joint of the tube 4. The inside diametral clearance in the spider hub 19 is greater than the CD. of the discharge pipe coupling 17 to permit the insertion of the completely assembled discharge pipe 7 into the separated segments of the water hammer tube 4. After the split collars 18 are individually inserted into the respective spider l9, and secured with snap ring lock 20, the mating flanges in the water hammer tube 4 are bolted together. The axial clearance between a typical pipe coupling 17 and the pipe guide assembly 18, 19, and 20, should be great enough to insure no axial interference during operational hammering shock and vibration.

The control valve poppet 12 is secured and positioned onto the valve stem 21 by a threaded flange 22. Flange 22 is bolted to the poppet 12 and threaded onto the stem 21. The flange threads are axially split and tightened onto the stem by bolted clamp 23 so as to eliminate any backlash between the threads. The control valve stem 21 is tightly fastened to the pneumatic actuating piston 24 by bolting, welding, or screwing depending upon the composition of the two respective materials. For example, if the stem is stainless steel and the piston is aluminum, one method of assembly is to thread both pieces and heat-shrink the aluminum onto the steel, then depending upon the differential in the coefficients of thermal expansion for disassembly. The valve stem 2] slides between a pair of bearings 25U and 25L conventionally secured in respective air cylinder end plates 26U and 26L. Protective housings 271) and 27L are pressure-compensated oil-filled (details not shown) to lubricate the valve stem 21. Elastomeric pressure seals 28, in various locations as shown, preclude respective fluid leakage. The pressure seals 28 on the piston 24 also may be piston rings. The conical control valve poppet 12 seats against a fitted seat 29 in the closed position and against elastomeric shock-absorbing bumpers 30 in the fully open position. The elastomeric pad 30 is supported by an annular plate welded to a six-legged spider 31. The valve body 8 is split in half axially along plane 14 to facilitate fabrication and maintenance. The air cylinder 33 is secured to the top flange of the control valve body 32 by the six-legged support 34 which provides minimum obstruction to the flow of water into the control valve 8.

The action of the air cylinder 9 is controlled by the electropneumatic valve 35 fastened immediately adjacent thereto. Only the output stage is represented here in a four-way valve type configuration which alternately connects the upper and lower air cylinder piston cavities 33U and 33L, respectively, to the air supply and discharge. The actuation of the three-land valve spool is represented by the double-pointed arrow at its upper end 36. This force may come from a springloaded electromagnet directly or via an intermediary pneumatic stage. This valve technology is well understood, is not limited to a three-land spool, and will not be detailed further. The filtered and dried, compressed air is circulated between the valve actuator and surface-located compressor and vacuum pump via corresponding feed and return lines in the power and control cable 11. These two cable loops respectively duct high pressure air chamber 37 and low pressure air chamber 38 which act as local reservoirs for the sudden demand.

In the air valve spool position shown in FIG. 3 high pressure air from chamber 37 is fed through line 40 into the lower air cylinder cavity 33L to close the water valve 8. Simultaneously, the low pressure chamber 38 receives the exhaust from the upper air cylinder cavity 33U along return line 41. When the water valve 8 is closed, the water evacuating pump 6 draws down the water level to that determined by the liquid level switch 39 in FIG. 2. At that time the electric signal is relayed to the electropneumatic valve actuator 36 to interchange the high and low pressure air connections to the air cylinder 33 causing the air piston to rapidly open the water control valve 8. The air pressure differential across the active area of the piston 24 exerts a force on the valve stem adequate to hold the poppet l2 tightly closed in its valve seat 29 under the ambient hydrostatic head during water pipe evacuation.

The rapid opening of the water control valve 8 also is aided by the force generated by the ambient hydrostatic head acting on the exposed area of the poppet valve. The inrushing water does not exert any drag forces on the pump 6 and motor 5 assembly because the liquid level switch 39 does not allow a water drawdown to the pump level. The casings of the pump, motor and discharge pipe can be made strong enough to withstand the resulting water hammer pressure. Problems resulting from the velocity shock generated by the rapid acceleration of the water hammer tube 4 in driving the pile can be overcome by making the motorpump-discharge pipe assembly free-floating and mechanically shock-isolated axially from the water hammer tube. This longitudinal shock isolation feature is provided by a lower compression spring 42 which supports the static air weight of the motor-pump-pipe assembly. An upper compression spring 43 helps the gravity return of the pump assembly to its normal midposition. A hydraulic type shock absorber 44 provides viscous damping to reduce oscillations. Further shock resistance is provided by making the motor-pump critical components neutrally buoyant in their respective liquid by means of low density construction materials and high density liquids incorporated in each respective frame. Hereby acceleration forces are transferred to the respective frames. The restoring force due to the upper compression spring is adjusted by the position of the split clamp 45. Teflon sliding shoes on 6 radial equispaced arms 46 fastened to the pump frame maintains its axis concentric within the water hammer tube. Three equispaced studs 47 fastened into the bottom flange 48 prevent the frames of the pump-motor assembly from rotating and also facilitate the torque-up of the discharge pipe couplings 17. Motor conductors 50 and liquid level switch leads 51 terminate in the watertight lower junction box 49 connecting with the power and control cable 11.

When the pile is of sufficient length and diameter, and also to facilitate the handling of long assemblies, the hammer tube 4 may be located internally within the pile as shown in FIG. 4, using any internal coupling, such as that (40) shown in FIG. 5. This permits incremental upward repositioning of the hammer as the pile l is driven into the bottom 100. It also permits coupling of the driver to the pile at a position which is closer to the sub-soil 100 than the top of the pile, giving an improved driving action. Concentrically alignment rings 52 may be secured to each water hammer tube flange joint as shown in FIG. 4.

To secure the hammer within the pile high pressure fluid is fed into the lower cylindrical cavity 53 of the pile coupler 40 through port 54, FIG. 5. This flow causes the cylinder frame 55 to move downward over the piston 56. The piston shaft is secured to the base 57 that is bolted to the bottom flange 48 of the water hammer tube 4. When the cylinder frame 55 moves downward it creates a toggle action in the multiplicity of links 58. The resultant mechanical advantage varies as the cotangent of the angle between the link and the radial normal. Consequently, hard-tooth shoes 59 slide radially outward in T slot guides in the base (57) and bite into the pile walls. Simultaneously, the fluid in the upper cylindrical cavity 60 is exhausted through port 61. A 4-way electrically-controlled valve, similar to 35, and not repeated herein, controls the influx and efflux of the pressurized fluid, which may be hydraulic or air. To release the pile coupler, the influx and efflux ports on the piston base are interchanged by means of this four-way control valve action. The compression spring (62) in the upper cylinder 60 retracts the entire mechanism when the air pressure is off. This pile coupler also can be used in the end-drive configuration.

Operationally, the hammer cycle comprises a relatively slow water-evacuation phase and a relatively fast power stroke. The evacuation period, depending upon the water-tube volume and pumping rate, awaits the actuation of the liquid level switch 39 to open the water control valve 8 by switching the position of the electropneumatic valve 35. However, since the power period is of the order of one second or less, the control of the electro-pneumatic valve 35 may be set by an electronic timer, one-shot multivibrator or the like to return the control valve to the closed position after such period of time. Consequently, the electric motor driving the water-evacuating pump need not be shut down and restarted for each hammer cycle. Of course, the elevation of the liquid level switch must compensate for the water level draw-down during this short driving period. In accordance with the invention, the enclosure walls can be at least partly, or wholly, defined by the walls of the pile being driven, as disclosed by FIG. 6. It repre sents a configuration of a pile 1 driven underwater into the ground 100 with the aid ofa module which includes parts similar to the pile driver of FIG. 1, and which therefore bear like reference numerals.

Extending axially through the control valve 8 and actuator 9 is a discharge pipe 7 which communicates with and supports a pump 6 by the pump discharge outlet. The pump in turn supports an electric drive motor 5. The valve 8, actuator 9, pipe 7, pump 6, motor 5, and cap to which they are secured constitute a unitary module which can be mated temporarily, during driving, with each of a series of piles.

Cap 120 is in water-tight sealing engagement with the pile mouth 1 and may be provided if desired, with means for remote-controlled release, e.g., a latch and trip-wire (not shown) to the surface, for releasing the module from the pile when driving has been completed. A wire rope sling 10 is provided for lowering and lifting the module onto and off of the piles, and also conveys the power and control cable harness 11 thereto. The module is serviced by a barge 124 having a winch 125 and cable 126 for lifting and lowering the module and a drum 127 for paying out and winding up the cable harness.

When the module is mated to a pile, the pile performs the function of the hammer tube 4 of the FIG. 1 embodiment, and the operation is the same. In this case, the evacuatable enclosure is defined by the cylindrical walls of the pile 1 and the inner surface 121 of the pile tip 122. The enclosure is coupled with the tip through the wall material of the pile.

During the operation of the device, water is evacuated from that portion of the enclosure which is at or above the level of pump inlet 123 and discharged through the discharge pipe 7. Upon opening of the fast acting valve 8, water is drawn through valve inlet 12 and is accelerated inwardly along the axis of the pile by the ambient hydrostatic head. Because the valve is provided with a large opening, the mass of water moving along the axis of the pile substantially fills the cross-section of the evacuated portion of the enclosure. The water is unrestrained and therefore continues moving substantially independent of the pile until it is suddenly decelerated against the barrier provided by the inner surface 121 of the pile tip, the water being substantially at its theoretical bulk modulus when decelerated. In this connection, it should be noted that the mass of water can be decelerated against the barrier either directly or per se (if provision were made for completely emptying the pile before admitting the water) or indirectly, such as by contacting the water accumulated in the lower portion of the pile. As a result of such deceleration at the waters theoretical bulk modulus, the hydraulic kinetic energy is converted to a powerful. water hammer driving pulse for driving the pile into the sub-soil.

The pile hammer 3 can be freely modified, as desired. For instance, the motor 5 and/or pump 6 may be located outside hammer tube 4 or pile 1 (see FIG. 11) provided the pump inlet is in communication with the interior of the tube at a position spaced along the tube axis from water control valve 8. Hammers can be employed which have water control valves at both ends and controls which would permit driving along the tube axis in either direction. Configurations may be fabricated for horizontal driving.

Different kinds of valves also may be used. Such others include spring-loaded, hydraulically and electromagnetically actuated linear and rotary shear varieties; metal, plastic and elastomeric pinch-off forms, free jet and fluidic submerged jet and pressure-switched groups; and, change-of-state valving techniques. Specific models are identified as spool and gridiron, sliding and rotary shear valves; conventional globe, gate, plug, ball, balanced/eccentric-pivoted poppet and butterfly, and flapper valves; resilient sleeve hydraulically, pneu matically. and mechanically squeezed pinch valves; jet pipes; electroviscous and magnetoviscous forms.

Practically, the principal limitation on the generation of larger values of water hammer within a single pipe may be the circumferential tensile or hoop stress. As will be shown, the generation of water hammer at a 1,000 ft. depth in a two-foot diameter steel pipe requires a wall thickness of 2.34 inches in order to keep the stress down to 69,000 psi. While this is not an excessive working stress for modern alloy steels, it still exceeds structural grade ratings. Since the invention is normally used in limited access or restricted environments, a low safety factor can be employed. To avoid the use of excessively massive pipe walls, reinforcement in the form of filament winding or an axial series of external or internal spaced reinforcing rings is recommended. The distributed spring mass configuration of the latter also reduces the transonic velocity of wave propagation along the pipe. Two advantages follow, namely, reduction of hoop stress and increase of impulse duration. The use of pipe wall materials with a lower elastic modulus like aluminum or resinated fiberglass achieves a reduction in water hammer pressure by lowering the transonic velocity. For these to be fully effective under water, additional accoustic pressure release material like cork may be applied at the ambient water interface in order to preclude acoustic loading. Another method of reducing wall stresses, by slowing down the axial velocity, is to use a series of truncated cone baffles 70 and 71 as illustrated in FIG. 7 or to spiral the water in the tube by uni-directionally twisted or alternately twisted bundles of smaller diameter tubes or baffles. Thus, as shown in FIG. 8, the water hammer tube 4 of any of the preceding embodiments may be provided with a plurality of twisted tubes 72 within the tube 4 and extending axially of at least a portion of the tube which defines the evacuatable enclosure. Where the tube 4 includes a discharger pipe 7 or other equipment along its axis, the spiral tubes 72 may be arranged around or above them. Similarly, baffles 75 and 76 of opposite rotation may be used, as disclosed in FIG. 9. The longer travel path provided by these various means proportionally creates a longer pulse.

The water hammer intensity can be reduced by retarding the rate at which the valve goes from full closed to full open position.

Thus, for a given driving application (assuming a given depth, pile mass and soil conditions) it is possible to tailor the force-time characteristics of the water hammer impulses by a suitable selection of. the length and diameters of the water hammer tube, and the reinforcement and material of construction thereof. Also, one may employ the acoustic pressure release material, baffles and valve opening rate as discussed above. Thus, it will be seen that the method has far more flexibility than is provided by the conventional steam hammer.

When the water hammer tube is provided with a unidirectional helix, a component of mechanical torque and rotation can be generated by the checked angular momentum of the falling mass of water. This can increase the penetrating power of the pile driver in cer- 10 tain soils. The screw vs. the nail" action also improves a friction piles load bearing capacity, especially when the lead or helix angle is optimized for the soil conditions.

FIG. 10 is a schematic diagram of an embodiment in which the pump means for evacuating the water hammer tube 4 pressurizes that chamber with gas or vapor to force out the liquids and then, the gases are removed by vacuum means.

The water hammer control valve 8 may be of a triggerlatched swing check valve configuration. When the remote-controlled triggering mechanism 134 retracts the holding pin the influx of ambient water 130 into the empty chamber 4 under the hydrostatic head to the water surface 131, pivots the flapper 132 and coupled shaft 133 to swing open and assume the position shown dotted. Upon striking the residual water surface 136 on the barrier 121, the incoming water m0- mentum generates the water hammer impulse as previously described.

All valves are remotely controlled and normally closed. Operational fluid flow is indicated by arrows. The water discharge valve 137, to empty the chamber, opens to the ambient water body. The gas/vapor inlet valve 138 connects with the remote 3-way valve 139 which normally taps the high pressure air supply 140 for blowing out the water and then momentarily taps the high pressure steam supply 141 for blowing out the air. Simultaneously with this steam cycle, the vacuum control valve 142 connects the chamber with a remote vacuum pump 143 to help exhaust the air. The steam upon condensing aids the vacuum-generation process. The flapper 132 is returned and relatched to reclose the valve 8 by mechanism 144 externally rotating the shaft 133. Valve-closing mechanism 144 (not described) may be a torsional spring, electric, or hydraulic rotary motor. The height of the residual water surface 136 remaining on the water hammer impact barrier 121 is a power-amplitude control means and is water readmitted by a time-calibrated reopening of valve 137.

FIG. 11 is a schematic diagram of an embodiment of the invention in which the inlet of the valve means is connected to a pressurized water reservoir 145 mounted atop the water hammer tube. Such a closed system configuration extends the operational milieu of the water hammer pile driver to offshore piles 1 whose tops extend above the water surface and also to landbased piles. The increased hydrostatic pressure increases the power rating of the water hammer. Although the gas/vapor/vacuum pumping technique previously described could be used herein, to illustrate diversification, a circumferential array of hollowshafted motor'driven (5) pumping (6) units is deployed externally. Arrows indicate operational fluid flow. The water level 131 in the reservoir is maintained via water supply valve 148 and a gas pressure on this water surface is maintained via valve 147. Although this figure illustrates a down-driving orientation, one can, employing the elastomeric membrane liquid/gas barrier common in the hydraulic accumulator art, allow water hammer operation in any orientation. Furthermore, such art shows that pressure on the'water surface also maybe applied by spring-loaded pistons and precharged gasloaded pistons. 1

Again for diversification, the swing-valve return mechanism 144 is herein illustrated as a linear motor which ram rod retracts prior to triggering the latch 135.

The check valves 146 in the water return lines 150 permit fluid flow only in the direction showed by the adjacent arrows so as to preserve the impact water level 136 in case it is desired to shut down the pumps for intermittent operation. The location of the water exhaust port 149 may be anywhere through the cylindrical walls of the chamber 4 and even adjacent the main control valve 8 for upwards driving.

For practical maintenance, shut-off valves would be placed in the pump (6) intake line at 151. However, for the purpose of illustrating a principal feature of the water hammer phenomenon, such a valve is unnecessary to prevent a hydraulic pressure short-circuit because the transonic travel time along the flooded return line 150 is practically the same as that along the shock tube 4. However, again for practical purposes it may be desirable to have a remote-operated shut-off valve at 151 to prevent the water hammer pressure from reaching the pump case 6.

Some contemporary offshore foundation designs call for loads up to 2000 tons from piles 200-600 ft. long, 3-8 ft. in diameter, weighing 100-200 tons, in up to 1000 ft. of water. Without supplementary techniques involving pre-drilling or jetting, such piles are practically undrivable by the steam-air hammer even when spliced to extend to the surface.

From the foregoing, it may be seen that the invention provides many advantages. It makes feasible a large increase in driving capability. And, this can be done using a smaller mass ratio (driving mass versus pile) than has heretofore been thought advisable in steam hammer operations. That is, larger impulses can be generated using a driving mass which is less than one-fourth that of the pile. This, in turn, makes it possible to drive piles without the use of palliatives such as pilot hole drilling, water jetting and grouting into an oversized hole, which measures can reduce pile load-bearing capacity.

The pressure-time characteristics of the water hammer impulse can be tailored over a wide range of values to match corresponding requirements of the pile and soil. Thus, driving impedance can be better matched to that of the earth than when operating with for instance a steel hammer.

Under the longer impulses which can be generated with a water ram or spear having a length to diameter ratio of 10 or greater, piles move more nearly as a unit, e.g., their driving action is more like that of a nail, rather than a worm, in which one part moves ahead while other parts are held back. Thus, a greater fraction of the driving energy is usefully expended in overcoming displacement skin friction, to advance the pile, rather than being tied up in the rubber-like ground quake. Unwanted reflected wave conditions in the pile can be prevented more effectively. The use of a cushion block, as sometimes required with a steel hammer, is unnecessary, thereby eliminating the inelastic collision energy loss associated therewith.

Certain important advantages are associated with the convenient manner in which the invention may be applied under water. With the driver submerged, it may be handled with greater safety and ease during storm conditions. Coupling of the driver to the pile at a point below itstop end helps to reduce losses of driving energy attributable to the mechanical compliance of the pile. Submerged operation provides inherent capacity for generating larger pulses as submergence increases, and particularly at depths greater than 200 feet where hydrostatic back pressure aggravates the venting problem of the air operated hammer, Where thermal line losses preclude the steam driven hammer, and where conventional vibratory driving requires such a relatively large back-mass for preload and such low frequencies that reaction forces necessary for driving become ineffective without excessively large excursions. Handling is facilitated because the driving mass can be drained from the apparatus when it is being transported and lifted above the surface.

Moreover, water hammer operation makes it convenient to twist the pile as it is driven downward, such as by including helical baffles in the water hammer tube which impart a twisting motion thereto. In some cases, especially where the lead or helix angle is optimized for the soil conditions, this can improve the piles load bearing capacity.

Although the invention has been referred to as including the use of a water hammer, liquids of densities greater or less than water, such as mercury or carbon tetrachloride, any be used in the hammer tube to achieve proportionately greater or lesser mechanica impulses.

In view of the foregoing, it is apparent that the present invention is a broad one, and that many changes may be made in the foregoing embodiments without departing from the spirit of the invention.

What is claimed is:

1. In the driving of a pile in an ambient fluid, including water or air, said pile having its tip embedded in the ground, a closed evacuatable enclosure with side walls and a barrier at one end being effectively coupled with the tip of the pile for transmitting driving forces exerted upon said barrier to said tip, the method which comprises: pumping out of said enclosure liquid, and at least a portion of any gases and vapors which may be present therein, for evacuating said enclosure sufficiently to provide space for acceleration and deceleration of a mass of liquid adequate to produce the necessary force and energy for driving said pile; maintaining said enclosure closed to said ambient fluid at least during said evacuation;

opening valve means through the wall of said enclosure; accelerating through said opening a substantially load-velocity independent flow of liquid into the evacuated portion of the enclosure; causing said flow to move toward said barrier with its principal component of energy and motion along the longitudinal axis of the enclosure toward the barrier for impacting against same directly or indirectly; suddenly decelerating said mass against said barrier with the liquid substantially at its theoretical bulk modulus, and thereby converting hydraulic kinetic energy to a liquid hammer driving pulse for driving said pile deeper into said subsoil; and repetitively evacuating, accelerating, decelerating and driving as aforesaid.

2. A method in accordance with claim 1 wherein the pile tip is embedded in the subsoil of a body of water.

3. A method in accordance with claim 1 wherein said evacuatable enclosure is submerged in said body of water.

4. A method in accordance with claim ll wherein said evacuatable enclosure is coupled to said pile below the top of said pile. 

1. In the driving of a pile in an ambient fluid, including water or air, said pile having its tip embedded in the ground, a closed evacuatable enclosure with side walls and a barrier at one end being effectively coupled with the tip of the pile for transmitting driving forces exerted upon said barrier to said tip, the method which comprises: pumping out of said enclosure liquid, and at least a portion of any gases and vapors which may be present therein, for evacuating said enclosure sufficiently to provide space for acceleration and deceleration of a mass of liquid adequate to produce the necessary force and energy for driving said pile; maintaining said enclosure closed to said ambient fluid at least during said evacuation; opening valve means through the wall of said enclosure; accelerating through said opening a substantially load-velocity independent flow of liquid into the evacuated portion of the enclosure; causing said flow to move toward said barrier with its principal component of energy and motion along the longitudinal axis of the enclosure toward the barrier for impacting against same directly or indirectly; suddenly decelerating said mass against said barrier with the liquid substantially at its theoretical bulk modulus, and thereby converting hydraulic kinetic energy to a liquid hammer driving pulse for driving said pile deeper into said subsoil; and repetitively evacuating, accelerating, decelerating and driving as aforesaid.
 2. A method in accordance with claim 1 wherein the pile tip is embedded in the subsoil of a body of water.
 3. A method in accordance with claim 1 wherein said evacuatable enclosure is submerged in said body of water.
 4. A method in accordance with claim 1 wherein said evacuatable enclosure is coupled to said pile below the top of said pile. 